Dynamics of a crystal containing a molecular impurity. I. Rigid molecule approximation

Abstract

The dynamics of a crystal containing a rigid, isolated substitutional molecular impurity is discussed using the Green's function method. The dynamical matrix for the problem is set up, and the various constraints on the force constants are pointed out. The application of the matrix partitioning technique is then indicated, and the possibility of a resonance mode arising from molecular librations is pointed out. Comparison is made with the earlier work of Wagner, and finally the relevance of the present formalism to the interpretation of a recent neutron scattering experiment is discussed.